This invention relates to catalytic methanation or hydrogenation of carbon oxides to methane and to a catalyst system for effecting same. More particularly, this invention is directed to a methanation process carried out at high temperatures and a molten metal salt-based heterogeneous catalyst system which is uniquely suited for such high temperature methanation.
Catalytic methanation is a well-known reaction which is widely employed in the chemical and energy providing industries. Probably its most widespread current and potential application is in the treatment of the gaseous effluent from the gasification or partial oxidation of carbonaceous fuels with oxygen and/or water, e.g., steam-hydrocarbon reforming and partial combustion of liquid and solid carbonaceous fuels, to produce a hydrogen-rich gas for chemical synthesis, e.g., ammonia manufacture, or petroleum refining, e.g., catalytic hydrocracking and hydrogenation, or to form a methane-rich gas having high BTU value and low CO content for use in residential and industrial heating or power generation. In the former case, the gasification or partial oxidation effluent, which typically contains substantial quantities of H.sub.2, CO, CO.sub.2 and H.sub.2 O as well as N.sub.2 in cases where air is used as the oxidant source, is generally subject to a process known as the carbon-monoxide shift-conversion reaction prior to catalytic methanation. In this case the CO-shift reaction converts a substantial quantity of the CO present to H.sub.2 and CO.sub.2 by reaction with H.sub.2 O in the presence of a catalyst and the primary purpose of catalytic methanation is to remove small quantities of CO which remain in the hydrogen-rich product gas by conversion to methane in order to avoid poisoning of downstream processing catalysts. In the latter case, i.e., conversion of partial oxidation effluent gas to methane-rich gas, the gasification or partial oxidation effluent gas is subject to CO-shift to obtain the appropriate ratio of H.sub.2 to CO (usually 3 to 1) and the CO shift product gas is then subject to catalytic methanation for conversion of carbon oxides and hydrogen contained therein to methane. In either case, the CO-shift effluent gas is subject to an intermediate processing step to remove sulfurous materials in cases where a sulfur-containing carbonaceous fuel feedstock is employed since all commercially used methanation catalysts are highly sensitive to poisoning by sulfur compounds.
Because of the increasing demand for a high BTU, clean gas as an energy source in the United States and the acknowledged decreasing and finite nature of natural gas reserves in the United States as well as happenings on the world scene which make energy self-sufficiency desirable or even essential, there has been a dramatic increase in interest in the manufacture of a clean, high BTU gas energy source which will meet pipeline standards by synthetic means from alternative carbonaceous resources such as coal or heavy hydrocarbons. Many of the more attractive synthetic approaches which have been proposed are based on gasification or partial combustion of the carbonaceous material, and, as indicated above, include catalytic methanation as part of the integrated process scheme to upgrade the BTU value of the product gas to a level acceptable for pipeline gas applications. CO and H.sub.2 have heating values of about 300 BTU/ft.sup.3 whereas pipeline natural gas has a value above 1000 BTU/ft.sup.3. While a number of metallic species are known to be active and selective methanation catalysts including, inter alia, nickel, ruthenium, cobalt, iron and molydenum, their application to the manufacture of high BTU or pipeline gas has been less than satisfactory for several reasons which relate to the physical form of the catalyst employed and/or the nature of the methanation reaction, itself.
In the first place, the primary thrust of previous efforts to effect catalytic methanation has been to utilize the active catalyst in solid form as a finely divided particulate on a refractory support, i.e., nickel on alumina or kieselguhr being pre-eminent, or as an alloy in a fixed or fluidized bed. These catalyst types are highly susceptible to inactivation via carbon deposition which can only be partially remedied by operation at undesirably high H.sub.2 /CO mole ratios in the feed gas. Furthermore, methanation reactions with these catalyst systems generally must be limited to tempertures below 400.degree. C. to avoid sintering and deactivation of the catalyst and the highly exothermic nature of the methanation reaction itself provides severe operational difficulties in controlling catalyst temperature in a fixed or fluidized bed at these levels when the CO concentration of the feed gas is in the range required for methane-rich gas manufacture. As an aside the use of the fixed or fluidized bed catalyst processing techniques also make it extremely difficult to recover any substantial quantity of the heat generated in the methanation for use in other phases of the process, e.g., the endothermic gasification in steam gasification of coal. Finally, the methanation reaction itself, is considered to be a combination of several reactions including the primary reaction (1) EQU 3 H.sub.2 +CO .fwdarw. CH.sub.4 +H.sub.2 O (1)
and secondary reactions (2) and (3) EQU 2 H.sub.2 +2CO .fwdarw. CH.sub.4 +CO.sub.2 ( 2) EQU 4 h.sub.2 +co.sub.2 .fwdarw. ch.sub.4 +2h.sub.2 o (3)
whose thermodynamic equilibria are such that the equlibriun yield of methane is adversely effected at high temperatures, i.e., above 500.degree. C.; reaction (2) being a combination of reaction (1) and the water gas shift reaction (4) EQU CO+H.sub.2 O .fwdarw. CO.sub.2 +H.sub.2 ( 4)
thus with conventional catalyst systems, methanations have been limited to the lowest temperatures consistent with acceptable catalyst activity in part, because of catalyst instability at high temperatures, the highly exothermic nature of the methanation reaction and the inability to effect an equilibrium shift towards methane, e.g., by absorption of one of the reaction products, at high temperatures under practical circumstances. A good review of previous efforts in catalytic methanation and the problems associated therewith can be found in G. A. Mill et al, "Catalytic Methanation", Catalysis Reviews, 8 (2), 159-210 (1973).
Accordingly, it would be desirable if an active catalyst system for methanation at temperatures above 500.degree. C. could be developed which would minimize operational problems associated with high temperature operation of the solid, particulate catalysts of the prior art, e.g., carbon deposition, instability and heat removal, while at the same time somehow shifting the methanation equilibrium towards methane formation, e.g., by H.sub.2 O absorption from the reaction mass, at these high temperatures. This would be espcially advantageous when catalytic methanation is utilized in conjunction with, for example, steam gasification of coal for the production of methane-rich gas. This is because the coal gasification reaction is high temperature but endothermic, thus requiring substantial input of high temperature heat such as that which could be recovered from an exothermic methanation reaction carried out at high temperatures. Furthermore, the reaction effluent from such coal gasification is many times already at or close to the thermodynamic equilibrium concentration of methanation reactants in a high temperature methanation reaction scheme, due to the high steam concentration of the gaseous effluent, and as such cannot be catalytically promoted towards methane formation unless one of the reaction products, particularly H.sub.2 O, is absorbed out of the reaction mass during or prior to methanation.